Free R

Free R is a statistical quantity introduced in 1992 by Axel T. Brünger to assess the quality of a model from X-ray crystallographic data. It is calculated in the same manner as the R value, but from a subset of the data set aside for the calculation of free R, and not used in the refinement of the model. It is a more reliable tool for assessing the model than the R value because it is not self-referential -- that is, as an estimation of errors, free R is free of any bias that may have been introduced during refinement.

As a rule of thumb, free R should not exceed the R value by more than 0.05; that is, if the R value is 0.20, free R should not significantly exceed 0.25. Free R values exceeding 0.40 raise serious doubts about the model.

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The initial contents of this page were adapted from the Glossary of ProteinExplorer.Org, with the permission of the principal author, Eric Martz. The Free R entry there was written primarily by Diana Ditmore.